arm64: Use the PAN msr mnemonic rather than .inst

Switch from creating the msr instructions to manage PAN to use the
"msr pan, #1" instruction directly. When this was added clang didn't
have support to assemble the instructions. This appears to have been
added to clang 13 which is sufficiently old enough.

Binutils releases from around the same time appear to have added this
instruction so any modern gcc should also support this instruction.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47817